1. Field of the Invention
The present invention relates to an antiskid control apparatus for a vehicle which performs antiskid control for preventing excessive slip of wheels (hereinafter referred to as “ABS control”).
2. Description of the Related Art
In general, an antiskid control apparatus includes a normally-open solenoid valve (pressure-increasing valve) interposed in a hydraulic circuit between the wheel cylinder and a master cylinder capable of generating brake hydraulic pressure (hereinafter referred to as “master cylinder pressure”) corresponding to a driver's brake operation; and a normally-closed solenoid valve (pressure-reducing valve) interposed in a hydraulic circuit between the wheel cylinder and a reservoir.
In general, ABS control is started when predetermined ABS-control start conditions are satisfied, and is accomplished by performing at least pressure-reducing control, and then pressure-increasing control. And the ABS control is continuously performed a plurality of times over a plurality of control cycles.
Incidentally, in recent years, there has arisen demand for control in which the wheel cylinder pressure is increased smoothly (seamlessly) during the above-mentioned pressure-increasing control (hereinafter referred to as “linear pressure-increasing control”). Therefore, some antiskid control apparatuses employ a linear solenoid valve (in particular, a normally-open linear solenoid valve) as the above-mentioned pressure-increasing valve. Such a linear solenoid valve enables the differential pressure (hereinafter referred to as “actual differential pressure”) obtained by subtracting the wheel cylinder pressure from the master cylinder pressure to be seamlessly adjusted by means of linearly controlling the current supplied to the pressure-increasing valve (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-19952).
In general, in a normally-open linear solenoid valve, proportionality exists between supplied current (instruction current) and differential pressure corresponding to attraction force (hereinafter referred to as “instruction differential pressure”). Accordingly, a normally-open linear solenoid valve, which serves as a pressure-increasing valve, closes when the instruction differential pressure determined from the supplied current is greater than the actual differential pressure, to thereby break the communication between the master cylinder and the wheel cylinder, and opens when the instruction differential pressure is less than the actual differential pressure, to thereby establish the communication between the master cylinder and the wheel cylinder.
Meanwhile, when the instruction differential pressure is less than the actual differential pressure, brake fluid flows from the master cylinder side into the wheel cylinder, whereby the wheel cylinder pressure increases, and the actual differential pressure decreases. When the actual differential pressure becomes equal to the instruction differential pressure, the actual differential pressure attains a balance with the instruction differential pressure.
That is, in order to smoothly increase the wheel cylinder pressure immediately after the linear pressure-increasing control is started by use of a normally-open linear solenoid valve serving as a pressure-increasing valve, the current supplied to the normally-open linear solenoid valve (pressure-increasing valve) must be controlled, with the pressure-reducing valve maintained closed, in such a manner that the current supplied to the normally-open linear solenoid valve is immediately set to a current value corresponding to the actual differential pressure (that is, a supply current value for rendering the instruction differential pressure coincident with the actual differential pressure; hereinafter referred to as “actual-differential-pressure corresponding current value”) at the start of the linear pressure-increasing control, and the supply current value is linearly decreased at a constant gradient. By virtue of this control, the actual differential pressure smoothly decreases from the start of the linear pressure-increasing control, which enables the wheel cylinder pressure to smoothly increase throughout the linear pressure-increasing control.
In contrast, in the case where the supply current value, which decreases throughout the linear pressure-increasing control, is set to a value greater than the actual-differential-pressure corresponding current value at the start of the linear pressure-increasing control, the normally-open linear solenoid valve is maintained in its closed state and the wheel cylinder pressure is held from the start of the linear pressure-increasing control until the decreasing instruction differential pressure becomes equal to the actual differential pressure. Herein, this phenomenon will be called “delay in starting of wheel cylinder pressure increase.”
Meanwhile, in the case where the supply current value, which decreases throughout the linear pressure-increasing control, is set to a value less than the actual-differential-pressure corresponding current value at the start of the linear pressure-increasing control, there arises a problem in that the normally-open linear solenoid valve is maintained in its opened state and the wheel cylinder pressure abruptly increases until the actual differential pressure, which decreases because of a flow of brake fluid from the master cylinder side into the wheel cylinder, becomes equal to the instruction differential pressure. Herein, this phenomenon will be called “abrupt increase of wheel cylinder pressure.”
Accordingly, in order to smoothly increase the wheel cylinder pressure immediately after the start of the linear pressure-increasing control, the actual-differential-pressure corresponding current value at the start of the linear pressure-increasing control (that is, the actual differential pressure at that time point) must be precisely determined. The actual differential pressure can be readily detected by use of a sensor for detecting the master cylinder pressure and a sensor for detecting the wheel cylinder pressure. However, the configuration utilizing such two sensors is not suitable for general employment. Therefore, the brake hydraulic pressure control apparatus disclosed in the above-mentioned publication, which performs linear pressure-increasing control, is designed to set the supply current value at the start of the linear pressure-increasing control to its maximum value in a first control cycle (first-time ABS control).
By virtue of this design, the instruction differential pressure at the start of the linear pressure-increasing control in the first control cycle becomes greater than the actual differential pressure without fail.
As a result, the actual differential pressure (=the instruction differential pressure) during the linear pressure-increasing control after the time point at which the instruction differential pressure has reached the actual differential pressure can be obtained. The apparatus disclosed in the above-mentioned publication is configured to obtain the actual differential pressure during a second or subsequent control cycle on the basis of the thus-obtained actual differential pressure.
However, in this case, there arises a problem in that the above-described “delay in starting of wheel cylinder pressure increase” always occurs in the linear pressure-increasing control in the first control cycle. According, desire has arisen for an alternative method which can properly estimate the actual differential pressure during ABS control.